Helium

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Helium

Background

Helium is one of the basic chemical elements. In its natural state, helium is a colorless gas known for its low density and low chemical reactivity. It is probably best known as a non-flammable substitute for hydrogen to provide the lift in blimps and balloons. Because it is chemically inert, it is also used as a gas shield in robotic arc welding and as a non-reactive atmosphere for growing silicon and germanium crystals used to make electronic semiconductor devices. Liquid helium is often used to provide the extremely low temperatures required in certain medical and scientific applications, including superconduction research.

Although helium is one of the most abundant elements in the universe, most of it exists outside of Earth's atmosphere. Helium wasn't discovered until 1868, when French astronomer Pierre Janssen and English astronomer Sir Joseph Lockyer were independently studying an eclipse of the Sun. Using spectrometers, which separate light into different bands of color depending on the elements present, they both observed a band of yellow light that could not be identified with any known element. News of their findings reached the scientific world on the same day, and both men are generally credited with the discovery. Lockyer suggested the name helium for the new element, derived from the Greek word helios for the sun.

In 1895, English chemist Sir William Ramsay found that cleveite, a uranium mineral, contained helium. Swedish chemists P.T. Cleve and Nils Langlet made a similar discovery at about the same time. This was the first time helium had been identified on Earth. In 1905, natural gas taken from a well near Dexter, Kansas, was found to contain as much as 2% helium. Tests of other natural gas sources around the world yielded widely varying concentrations of helium, with the highest concentrations being found in the United States.

During the early 1900s, the development of lighter-than-air blimps and dirigibles relied almost entirely on hydrogen to provide lift, even though it was highly flammable. During World War I, the United States government realized that non-flammable helium was superior to hydrogen and declared it a critical war material. Production was tightly controlled, and exports were curtailed. In 1925, the United States passed the first Helium Conservation Act which prohibited the sale of helium to nongovernmental users. It wasn't until 1937, when the hydrogen-filled dirigible Hindenburg exploded while landing at Lakehurst, New Jersey, that the restrictions were lifted and helium replaced hydrogen for commercial lighter-than-air ships.

During World War II, helium became a critical war material again. One of its more unusual uses was to inflate the tires on long-range bomber aircraft. The lighter weight of helium allowed the plane to carry 154 lb (70 kg) of extra fuel for an extended range.

After the war, demand for helium grew so rapidly that the government imposed the Helium Act Amendments in 1960 to purchase and store the gas for future use. By 1971, the demand had leveled off and the helium storage program was canceled. A few years later, the government started storing helium again. As of 1993, there were
about 35 billion cubic feet (1.0 billion cubic meters) of helium in government storage.

Today, the majority of the helium-bearing natural gas sources are within the United States. Canada, Poland, and a few other countries also have significant sources.

Raw Materials

Helium is generated underground by the radioactive decay of heavy elements such as uranium and thorium. Part of the radiation from these elements consists of alpha particles, which form the nuclei of helium atoms. Some of this helium finds its way to the surface and enters the atmosphere, where it quickly rises and escapes into space. The rest becomes trapped under impermeable layers of rock and mixes with the natural gases that form there. The amount of helium found in various natural gas deposits varies from almost zero to as high as 4% by volume. Only about one-tenth of the working natural gas fields have economically viable concentrations of helium greater than 0.4%.

Helium can also be produced by liquefying air and separating the component gases. The production costs for this method are high, and the amount of helium contained in air is very low. Although this method is often used to produce other gases, like nitrogen and oxygen, it is rarely used to produce helium.

The ManufacturingProcess

Helium is usually produced as a byproduct of natural gas processing. Natural gas contains methane and other hydrocarbons, which are the principal sources of heat energy when natural gas is burned. Most natural gas deposits also contain smaller quantities of nitrogen, water vapor, carbon dioxide, helium, and other non-combustible materials, which lower the potential heat energy of the gas. In order to produce natural gas with an acceptable level of heat energy, these impurities must be removed. This process is called upgrading.

There are several methods used to upgrade natural gas. When the gas contains more than about 0.4% helium by volume, a cryogenic distillation method is often used in order to recover the helium content. Once the helium has been separated from the natural gas, it undergoes further refining to bring it to 99.99+% purity for commercial use.

Here is a typical sequence of operations for extracting and processing helium.

Pretreating

Because this method utilizes an extremely cold cryogenic section as part of the process, all impurities that might solidify—such as water vapor, carbon dioxide, and certain heavy hydrocarbons—must first be removed from the natural gas in a pretreatment process to prevent them from plugging the cryogenic piping.

1 The natural gas is pressurized to about 800 psi (5.5 MPa or 54 atm). It then flows into a scrubber where it is subjected to a spray of monoethanolamine, which absorbs the carbon dioxide and carries it away.

2 The gas stream passes through a molecular sieve, which strips the larger water vapor molecules from the stream while letting the smaller gas molecules pass. The water is back-flushed out of the sieve and removed.

3 Any heavy hydrocarbons in the gas stream are collected on the surfaces of a bed of activated carbon as the gas passes through it. Periodically the activated carbon is recharged. The gas stream now contains mostly methane and nitrogen, with small amounts of helium, hydrogen, and neon.

Separating

Natural gas is separated into its major components through a distillation process known as fractional distillation. Sometimes this name is shortened to fractionation, and the vertical structures used to perform this separation are called fractionating columns. In the fractional distillation process, the nitrogen and methane are separated in two stages, leaving a mixture of gases containing a high percentage of helium. At each stage the level of concentration, or fraction, of each component is increased until the separation is complete. In the natural gas
industry, this process is sometimes called nitrogen rejection, since its primary function is to remove excess quantities of nitrogen from the natural gas.

4 The gas stream passes through one side of a plate fin heat exchanger while very cold methane and nitrogen from the cryogenic section pass through the other side. The incoming gas stream is cooled, while the methane and nitrogen are warmed.

5 The gas stream then passes through an expansion valve, which allows the gas to expand rapidly while the pressure drops to about 145-360 psi (1.0-2.5 MPa or 10-25 atm). This rapid expansion cools the gas stream to the point where the methane starts to liquefy.

6 The gas stream—now part liquid and part gas—enters the base of the high-pressure fractionating column. As the gas works its way up through the internal baffles in the column, it loses additional heat. The methane continues to liquefy, forming a methane-rich mixture in the bottom of the column while most of the nitrogen and other gases flow to the top.

7 The liquid methane mixture, called crude methane, is drawn out of the bottom of the high-pressure column and is cooled further in the crude subcooler. It then passes through a second expansion valve, which drops the pressure to about 22 psi (150 kPa or 1.5 atm) before it enters the low-pressure fractionating column. As the liquid methane works its way down the column, most of the remaining nitrogen is separated, leaving a liquid that is no more than about 4% nitrogen and the balance methane. This liquid is pumped off, warmed, and evaporated to become upgraded natural gas. The gaseous nitrogen is piped off the top of the low-pressure column and is either vented or captured for further processing.

8 Meanwhile, the gases from the top of the high-pressure column are cooled in a
condenser. Much of the nitrogen condenses into a vapor and is fed into the top of the low-pressure column. The remaining gas is called crude helium. It contains about 50-70% helium, 1-3% unliquefied methane, small quantities of hydrogen and neon, and the balance nitrogen.

Purifying

Crude helium must be further purified to remove most of the other materials. This is usually a multi-stage process involving several different separation methods depending on the purity of the crude helium and the intended application of the final product.

9 The crude helium is first cooled to about -315° F (-193° C). At this temperature, most of the nitrogen and methane condense into a liquid and are drained off. The remaining gas mixture is now about 90% pure helium.

10 Air is added to the gas mixture to provide oxygen. The gas is warmed in a preheater and then it passes over a catalyst, which causes most of the hydrogen in the mixture to react with the oxygen in the air and form water vapor. The gas is then cooled, and the water vapor condenses and is drained off.

11 The gas mixture enters a pressure swing adsorption (PSA) unit consisting of several adsorption vessels operating in parallel. Within each vessel are thousands of particles filled with tiny pores. As the gas mixture passes through these particles under pressure, certain gases are trapped within the particle pores. The pressure is then decreased and the flow of gas is reversed to purge the trapped gases. This cycle is repeated after a few seconds or few minutes, depending on the size of the vessels and the concentration of gases. This method removes most of the remaining water vapor, nitrogen, and methane from the gas mixture. The helium is now about 99.99% pure.

Distributing

Helium is distributed either as a gas at normal temperatures or as a liquid at very low temperatures. Gaseous helium is distributed in forged steel or aluminum alloy cylinders at pressures in the range of 900-6,000 psi (6-41 MPa or 60-410 atm). Bulk quantities of liquid helium are distributed in insulated containers with capacities up to about 14,800 gallons (56,000 liters).

12 If the helium is to be liquefied, or if higher purity is required, the neon and any trace impurities are removed by passing the gas over a bed of activated carbon in a
cryogenic adsorber operating at about -423° F (-253° C). Purity levels of 99.999% or better can be achieved with this final step.

13 The helium is then piped into the liquefier, where it passes through a series of heat exchangers and expanders. As it is progressively cooled and expanded, its temperature drops to about -452° F (-269° C) and it liquefies.

14 Large quantities of liquid helium are usually shipped in unvented, pressurized containers. If the shipment is within the continental United States, shipping time is usually less than a week. In those cases, the liquid helium is placed in large, insulated tank trailers pulled by truck tractors. The tank body is constructed of two shells with a vacuum space between the inner and outer shell to retard heat loss. Within the vacuum space, multiple layers of reflective foil further halt heat flow from the outside. For extended shipments overseas, the helium is placed in special shipping containers. In addition to a vacuum space to provide insulation, these containers also have a second shell filled with liquid nitrogen to absorb heat from the outside. As heat is absorbed, the liquid nitrogen boils off and is vented.

Quality Control

The Compressed Gas Association establishes grading standards for helium based on the amount and type of impurities present. Commercial helium grades start with grade M, which is 99.995% pure and contains limited quantities of water, methane, oxygen, nitrogen, argon, neon, and hydrogen. Other higher grades include grade N, grade P, and grade G. Grade G is 99.9999% pure. Periodic sampling and analysis of the final product ensures that the standards of purity are being met.

The Future

In 1996, the United States government proposed that the government-funded storage program for helium be halted. This has many scientists worried. They point out that helium is essentially a waste product of natural gas processing, and without a government storage facility, most of the helium will simply be vented into the atmosphere, where it will escape into space and be lost forever. Some scientists predict that if this happens, the known reserves of helium on Earth may be depleted by the year 2015.

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Helium (revised)

HELIUM (REVISED)

Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.

Overview

Helium is a member of the noble gas family. The noble gases are the elements in Group 18 (VIIIA) of the periodic table. The periodic table is a chart that shows how the elements are related to one another. The noble gases are also called the inert gases. Inert means that an element is not very active. It will not combine with other elements or compounds. In fact, no compounds of helium have ever been made.

Helium is the second most abundant element in the universe. Only hydrogen occurs more often than helium. Helium is also the second simplest of the chemical elements. Its atoms consist of two protons, two neutrons, and two electrons. Only the hydrogen atom is simpler than a helium atom. The hydrogen atom has one proton, one electron, and no neutrons.

Helium was first discovered not on Earth, but in the Sun. In 1868 French astronomer Pierre Janssen (1824-1907) studied light from the Sun during a solar eclipse. He found proof that a new element existed in the Sun. He called the element helium.

SYMBOL
He

ATOMIC NUMBER
2

ATOMIC MASS
4.002602

FAMILY
Group 18 (VIIIA)
Noble gas

PRONUNCIATION
HEE-lee-um

Helium has some interesting and unusual physical properties. For example, at very low temperatures it can become superfluid.
A superfluid material behaves very strangely. It can flow upwards out of a container, against the force of gravity. It can also squeeze through very small holes that should be able to keep it out. The Nobel Prize in physics for 1996 was awarded to three Americans who discovered superfluidity. They were David M. Lee (1931- ), Douglas D. Osheroff (1945- ), and Robert C. Richardson (1937-).

For an inactive gas, helium has a surprising number of applications. It is used in low-temperature research, for filling balloons and dirigibles (blimps), to pressurize rocket fuels, in
welding operations, in lead detection systems, in neon signs, and to protect objects from reacting with oxygen.

Discovery and naming

One of the most powerful instruments for studying chemical elements is the spectroscope. The spectroscope is a device for studying the light produced by a heated object. For example, a lump of sodium metal will burn with a yellow flame. The flame looks quite different, however, when viewed through a spectroscope.

A spectroscope contains a triangular piece of glass (called a prism) that breaks light into its basic parts. These basic parts consist of a series of colored lines. In the case of sodium, the yellow light is broken into a series of yellow lines. These lines are called the element's spectrum. Every element has its own distinctive spectrum.

The spectroscope gives scientists a new way of studying elements. They can identify an element by recognizing its distinctive spectral lines even when they can't actually see the element itself.

This principle led to the discovery of helium. In 1868, Janssen visited India in order to observe a full eclipse of the Sun. A solar eclipse occurs when the Moon comes between the Sun and the Earth. The Moon blocks nearly all of the Sun's light. All that remains is a thin outer circle (corona) of sunlight around the dark Moon. Solar eclipses provide scientists with an unusual chance to study the Sun.

Janssen examined light from the Sun with a spectroscope. As he looked at the spectral lines, he was surprised to see some lines that could not be traced to any known element. He concluded that there must be an element on the Sun that had never been seen on Earth. The name helium was later suggested for this element. The name comes from the Greek word helios for "sun."

French astronomer Pierre Janssen discovered helium after studying the Sun during a full solar eclipse.

Chemists did not know what to make of Janssen's discovery. Was there an element on the Sun that did not exist on Earth? Had he made a mistake? Some scientists even made fun of Janssen.

For the next thirty years, chemists looked for helium on Earth. Then, in 1895, the English physicist Sir William Ramsay (1852-1916) found helium in a mineral of the element uranium. Credit for the earthly discovery of helium is sometimes given to two other scientists also. Swedish chemists Per Teodor Cleve (1840-1905) and Nils Abraham Langlet also discovered helium at about the same time in a mineral called cleveite.

Ramsay did not know why helium occurred in an ore of uranium. Some years later, the reason for that connection became obvious. Uranium is a radioactive element. A radioactive element is one that breaks apart spontaneously. It releases radiation and changes into a new element.

T hat's the last potato I'll ever dig!" That statement was attributed to young Ernest Rutherford, a native of New Zealand. Rutherford had applied for a scholarship to Cambridge University, one of England's most famous universities. Rutherford had actually finished second in the scholarship competition. But the winner had decided to stay in New Zealand and get married. When Rutherford was told he had won the scholarship, he threw down his potato fork and announced the end of his potato-digging days.

Rutherford went on to become one of the great scientific figures of the twentieth century. He made a number of important discoveries about the structure of atoms and about radioactivity. For example, he found that an atom consists of two distinct parts, the nucleus and the electrons. He also discovered one form of radiation given off by radioactive materials: alpha particles. Alpha particles, he found, are simply helium atoms without their electrons.

Rutherford was also the first scientist to change one element into another. He accomplished this by bombarding nitrogen gas with alpha particles. Rutherford found that oxygen was formed in this experiment. He had discovered a way to convert one element (nitrogen) into a different element (oxygen). The method Rutherford used later became a standard procedure used by many other scientists.

One form of radiation produced by uranium consists of alpha particles. Alpha particles are tiny particles moving at very high
rates of speed. In 1907, English physicist Ernest Rutherford (1871-1937) showed that an alpha particle is nothing more than a helium atom without its electrons. As uranium atoms broke apart, then, they gave off alpha particles (helium atoms). That is why helium was first found on Earth in connection with uranium ores.

Physical properties

Helium is a colorless, odorless, tasteless gas. It has a number of unusual properties. For example, it has the lowest boiling point of any element, -268.9°C (-452.0°F). The boiling point for a gas is the temperature at which the gas changes to a liquid. The freezing point of helium is -272.2°C (-458.0°F). Helium is the only gas that cannot be made into a solid simply by lowering the temperature. It is also necessary to increase the pressure on the gas in order to make it a solid.

At a temperature of about -271°C (-456°F), helium undergoes an unusual change. It remains a liquid, but a liquid with strange properties. Superfluidity is one of these properties. The forms of helium are so different that they are given different names. Above -271°C, liquid helium is called helium I; below that temperature, it is called helium II.

Chemical properties

Helium is completely inert. It does not form compounds or react with any other element.

Occurrence in nature

Helium is the second most abundant element after hydrogen in the universe and in the solar system. About 11.3 percent of all atoms in the universe are helium atoms. By comparison, about 88.6 percent of all atoms in the universe are hydrogen. Thus, at least 99.9 percent of all atoms are either hydrogen or helium atoms.

By contrast, helium is much less abundant on Earth. It is the sixth most abundant gas in the atmosphere after nitrogen, oxygen, argon,carbon dioxide, and neon. It makes up about 0.000524 percent of the air.

It is probably impossible to estimate the amount of helium in the Earth's crust. The gas is produced when uranium and other
radioactive elements break down. But it often escapes into the atmosphere almost immediately.

Isotopes

Two isotopes of helium occur naturally, helium-3 and helium-4. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.

Three radioactive isotopes of helium have been made also. A radioactive isotope is one that breaks apart and gives off some form of radiation. Radioactive isotopes are produced when very small particles are fired at atoms. These particles stick in the atoms and make them radioactive.

None of the radioactive isotopes of helium has any commercial application.

Extraction

In theory, helium could be collected from liquid air. Liquid air is air that has been cooled to a very low temperature. All of the gases in air have liquefied in liquid air. If the liquid air were allowed to evaporate, the last gas remaining after all other gases had evaporated would be helium. There is too little helium in air to make this process worthwhile, however.

There is a much better source of helium. The gas often occurs along with natural gas in reservoirs deep beneath the Earth's surface. When wells are dug to collect the natural gas, helium comes to the surface with the natural gas. Then, helium can be separated from natural gas very easily. The temperature of the mixture is lowered, and the natural gas liquefies and is taken away. Gaseous helium is left behind.

Helium often occurs along with natural gas in reservoirs deep beneath the Earth's surface.

About 80 percent of the world's helium is in the United States. In 1996, 20 U.S. plants produced helium from gas wells. About 86 percent of U.S. helium comes from five large underground regions: the Hugoton Field that lies beneath Kansas, Oklahoma, and Texas; the Keyes Field in Oklahoma; the Panhandle and Cliffside Fields in Texas; and the Ridley Ridge area in Wyoming.

For many years, the U.S. government operated the Federal Helium Program. This program was responsible for collecting and storing helium for government use. The main customers for this helium were the Department of Defense, the National Aeronautics and Space Administration, and the Department of Energy. The helium was stored underground in huge natural caves.

In 1996, the government decided to end this program. Helium was no longer regarded as essential to national security. The Bureau of Mines has begun to sell off the federal reserves.

Uses

The most important single use for helium in the United States is in low-temperature cooling systems. This is because liquid helium—at -270°C—s cold enough to cool anything else. For example, it is used in superconducting devices.

A superconducting material is one that has no resistance to the flow of electricity. Once an electric current begins to flow in the material, it will continue to flow forever. No energy is wasted in moving electricity from one place to another. Superconducting materials may revolutionize electrical systems worldwide someday. The problem is that superconductivity occurs only at very low temperatures. One way to achieve those temperatures is with liquid helium.

Another important use of helium is in pressure and purge systems. In many industrial operations, it is necessary to pressurize a system. The easiest way to do that is to pump a gas into the system. But the gas should not be one that will react with other substances in the system. Being inert, helium is a perfect choice. Helium is also used for purging, a process that sweeps away all gas in a container. Again, helium is used because it does not react with anything in the container.

Helium is used to inflate balloons and other lighter-than-air crafts, such as dirigibles (blimps).

Because of its inactivity, helium is also used in welding systems. Welding is the process by which two metals are heated to high temperatures in order to join them to each other. Welding rarely works well in "normal" air. At high temperatures, the metals may react with oxygen to form metal oxides. If they do, they are less likely to join to each other. If the welding is done in a container of helium, this is not a problem. The metals will not react with helium. They will simply join to each other.

Helium is also used in leak-detection systems. If a leak is suspected in a long pipe, helium can be used to look for that leak. It is pumped into one end of the pipe. A detector is held outside the pipe. The detector is designed to measure whether helium is escaping from the system. The detector is moved along the length of the pipe. It is possible to find out whether there is a leak, where it is, and how much it is leaking. Helium is a good gas to use for this purpose because it does not react with anything in the pipe.

Helium is also used to inflate balloons and other lighter-thanair crafts, such as dirigibles (blimps). Helium does not have the lifting power of hydrogen. However, hydrogen is flammable and helium is not. At one time, people thought that dirigibles would be a popular form of transportation. But that never happened. Blimps are still used for limited purposes, such as advertising at major sports and recreational events.

Compounds

No compounds of helium have ever been made.

Health effects

There are no known health hazards resulting from exposure to helium.

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Spectroscopic evidence for the presence of helium in the sun was first obtained during a solar eclipse in 1868. A bright yellow emission line was observed and was later shown to correspond to no known element; the new element was named by J. N. Lockyer and E. Frankland from helios [Gr.,=sun]. Helium was isolated (1895) from a sample of the uranium mineral cleveite by Sir William Ramsay.

Properties and Isotopes

Helium is less dense than any other known gas except hydrogen and is about one seventh as dense as air. Extremely unreactive, it is an inert gas in Group 18 of the periodic table. Natural helium is a mixture of two stable isotopes, helium-3 and helium-4. In helium obtained from natural gas about one atom in 10 million is helium-3. The unstable isotopes helium-5, helium-6, and helium-8 have been synthesized. The alpha particles that are emitted from certain radioactive substances are identical to helium-4 nuclei (two protons and two neutrons).

Helium-4 is unusual in that it forms two different kinds of liquids. When it is cooled below 4.22°K (its boiling point at atmospheric pressure) it condenses to liquid helium-I, which behaves as an ordinary liquid. When liquid helium-I is cooled below about 2.18°K (at atmospheric pressure), liquid helium-II is formed. Liquid helium-II has a number of unusual properties. It is sometimes called a superfluid because it has extremely low viscosity. It also has extremely high heat conductivity and expands on cooling. It cannot be contained in an open beaker since a thin film of it creeps up the side, over the lip, and flows down the outside. The study of these phenomena is a part of low-temperature physics. When helium-3 is liquefied and cooled it does not exhibit the properties of liquid helium-II; this difference in properties between helium-3 and helium-4 can be explained in terms of quantum mechanics.

Natural Occurrence and Preparation

Helium is rare and costly. Wells in Texas (where the Federal Helium Reserve was established in 1925 near Amarillo), Oklahoma, and Kansas are the principal world source. Crude helium is separated by liquefying the other gases present in the natural gas; it is then either further purified or stored for later purification and use. Some helium is extracted directly from the atmosphere; the gas is also found in certain uranium minerals and in some mineral waters, but not in economic quantities. It has been estimated that helium makes up only about 0.000001% of the combined weight of the earth's atmosphere and crust; it is most concentrated in the exosphere, which is the outermost region of the atmosphere, 600–1500 mi (960–2400 km) above the earth's surface. Helium is abundant in outer space; it makes up about 23% of the mass of the visible universe. It is the end product of energy-releasing fusion processes in stars (see interstellar matter).

Uses

Helium's noncombustibility and buoyancy (second only to hydrogen) make it the most suitable gas for balloons and other lighter-than-air craft. A mixture of helium and oxygen is often supplied as a breathing mixture for deep-sea divers and caisson workers and is used in decompression chambers; because helium is less soluble in human blood than nitrogen, its use reduces the risk of caisson disease, or the
"bends."
Helium can also be used wherever an unreactive atmosphere is needed, e.g., in electric arc welding, in growing crystals of silicon and germanium for semiconductors, and in refining titanium and zirconium metals. It is also used to pressurize the fuel tanks of liquid-fueled rockets. Liquid helium is essential for many low temperature applications (see low-temperature physics).

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Helium

Helium, a colorless gas at room temperature, is the first element in the noble gas group, and forms few compounds. It is rare in the atmosphere (1 part in 200,000) and recovered on Earth principally by its separation from natural gas obtained in underground wells. Named for the Sun (in Greek, helios ), helium is a component of the production of energy as well as the basis of the science and technology of cryogenics. Its presence at the surface of the Sun was first confirmed by amateur British astronomer Joseph Norman Lockyer (1868), who observed characteristic lines in the optical spectrum of the Sun, at whose surface helium is produced via the energy-releasing fusion of hydrogen and deuterium nuclei.

Because it is such a light, nonreactive element, helium condenses (at atmospheric pressure) only at 4.2 kelvins. Furthermore, because of quantum mechanical effects, helium solidifies (under the application of 25.3 bars of external pressure) only at the lowest temperatures. Liquefied in large compression refrigerators, helium is used to cool cryogenic equipment, in particular the superconducting magnets used in medical magnetic resonance imaging (MRI). At 2.17 kelvins liquid helium transforms into an unusual quantum phase , called a superfluid, which has no viscosity and exhibits bizarre flow properties, such as its creeping out of containers.

The gas is also used to fill balloons, in gas discharge lamps, and as an additive in the breathing gases of astronauts and scuba divers. The rarer stable isotope of helium (3He) is produced by the decay of radioactive tritium, and is used in resonance imaging and in the attainment of very low temperatures, about 0.010 kelvin, via a process known as dilution refrigeration.

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helium

helium (symbol He) Nonmetallic element, a noble gas, discovered in 1868 by Jules Janssen. First obtained in 1895 from the mineral clevite, the chief source today is from natural gas. It is also found in some radioactive minerals and in the Earth's atmosphere (0.0005% by volume). It has the lowest melting and boiling points of any element. It is colourless, odourless and nonflammable, and is used in light-air balloons, to make artificial ‘air’ (with oxygen) for deep-sea divers and in welding, semiconductors and lasers. Liquid helium is used in cryogenics (physics dealing with low temperatures). Properties: at.no. 2; r.a.m. 4.0026; r.d. 0.178; m.p. −272.2°C (−458°F); b.p. −268.9°C (−452.02°F); single isotope He4.

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Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

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Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

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Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

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Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

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Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.

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Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

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Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.

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